Well, this happens when the matrix does not have full rank, so, either you have no solutions, or you have an infinite number; in the case where you have one row of zeros, you will have one free variable in your solution set.

Enter 4 in the rows/columns box and then enter the coefficients with the answers in the last column. Then hit calculate. You get a complete row of zeroes at the bottom. The implications of this were explained by another poster.